1. Field of the Invention
The invention relates generally to the processing of decimal numeric instructions by a microprogrammed data processing system and more particularly to the stripping and appending of non-decimal digit characters to the operand.
2. Description of the Prior Art
Digital computer systems execute instructions calling for the processing of decimal operands. The decimal operands are made up of a plurality of words with each word containing a plurality of decimal characters. A word may also contain information that is not included in the operand. The decimal characters includes decimal digits being in the form of 4-bit decimal digits packed up to eight decimal digits per word, or 9-bit ASCII or EBCDIC decimal digits packed up to four decimal digits per word. The decimal characters also include sign characters and, for floating point instructions, exponent characters.
The operands are stored in memory and, in the prior art, are transferred to an arithmetic unit where the words are stripped of sign characters, exponent characters and non-operand characters. In addition, the 9 bit decimal digits are stripped of the zone bits and packed into words of up to eight decimal digits per word. This readily permits decimal arithmetic wherein operands containing 9-bit decimal digits are processed with 4-bit decimal digits in response to the instruction calling for a decimal arithmetic operation of the two operands.
Before the resulting operand is stored back in memory, however, the necessary sign, exponent and non-operand characters must be added, as well as the zone bits if the resulting operand is made up of 9-bit characters.
The Honeywell 6000 computing system performed the merging of sign, zone bits, exponent and non-operand characters along with the decimal digits into the words of the resulting operand using considerable combinational logic.
Microprogrammed computers perform this merging operation by microprogrammed subroutines shifting the decimal digits of the arithmetic result into the proper position within the word for and adding the sign, exponent, zone bits and non-operand information. Since the microprogrammed subroutines performed in a serial fashion, the system throughput was degraded.